Various arrangements for automatic irrigation of plants are well known to the art. U.S. Pat. No. 3,753,315 is illustrative of a double pot system in which an aperture is located on or near the bottom of an inner pot and a sponge located between the bottom of the inner container and above the inside bottom of an outer container. Water is transferred from the outer container to a cavity containing the sponge and, through capillary action, the water moves upwardly from the sponge through the aperture in the inner pot to moisten the plant soil. One problem associated with the device has been the difficulty of controlling the amount of water transferred to the plant. Quite often such types of plant watering systems have not proved satisfactory as soil often becomes water saturated and causes rotting of the plant roots.
Another form of automatic watering device is shown in U.S. Pat. No. 4,825,591 in which a plant hangs from one end of a vertical rod with a spring at the opposite end of the rod for actuating a valve in a water tank. As the soil dries and becomes lighter, the spring lifts the plant and triggers a water tank valve to allow water to flow to the plant. The complexity of this apparatus makes it susceptible to high failure rates and its size and complexity are not cost effective for mass watering.